1. Field of the Invention
The invention relates to a catadioptric projection objective for imaging a pattern arranged in the object plane of the projection objective into the image plane of the projection objective whilst generating at least one real intermediate image.
2. Description of the Related Art
Projection objectives of this type are used in microlithography projection exposure installations for fabricating semiconductor components and other finely structured devices. They serve for projecting patterns of photomasks or lined plates, generally referred to below as masks or reticles, onto an article coated with a light-sensitive layer with very high resolution on a demagnifying scale.
In this case, the production of ever finer structures necessitates, on the one hand, enlarging the numerical aperture (NA) of the projection objective on the image side and, on the other hand, using ever shorter wavelengths, preferably ultraviolet light having wavelengths of less than approximately 260 nm, for example 248 nm, 193 nm or 157 nm.
For these short wavelengths it becomes more and more difficult to provide purely refractive systems with adequate correction of chromatic aberrations, since the Abbe constants of suitable transparent materials are relatively close together. Therefore, for very high-resolution projection objectives use is made predominantly of catadioptric systems, in which refractive and reflective components, that is to say in particular lenses and mirrors, are combined.
When utilizing imaging mirror surfaces, it is necessary to use beam deflecting devices if obscuration-free and vignetting-free imaging is to be achieved. There are systems with geometrical beam deflection, in which the beam deflecting device has at least one fully reflective deflection mirror. Systems with physical beam deflection are also known, for example systems with a physical beam splitter in the form of a beam splitter cube (BSC). Although systems without an intermediate image are available, use is increasingly being made of projection objectives with an intermediate image, which permit greater structural freedoms with regard to the type and arrangement of lenses and other optical components.
The projection objectives are usually equipped with a system diaphragm for limiting the cross section of the radiation passing through the projection objective. The diaphragm diameter of the system diaphragm can preferably be set in a variable manner in order to be able to set, for a given application, the best compromise between numerical aperture or resolution, on the one hand, and depth of focus (DOF), on the other hand.
Such a system diaphragm, also called aperture diaphragm or aperture stop, is fitted in the vicinity of a suitable diaphragm location of the optical axis. Suitable diaphragm locations lie at or in the vicinity of axial positions in which the principal ray of the optical imaging intercepts the optical axis. Imaging systems with an intermediate image have two such diaphragm locations, namely one between object plane and intermediate image and one between the intermediate image and the image plane.
In known catadioptric projection objectives with an intermediate image, the system diaphragm is fitted within the dioptric objective part at the diaphragm location lying in the vicinity of the image plane. Examples of such systems are shown, inter alia, in EP 0 869 383 (corresponding to U.S. Pat. No. 5,969,882), U.S. Pat. No. 6,157,498 or U.S. Pat. No. 5,808,805 or the associated continuation application U.S. Pat. No. 5,999,333.
U.S. Pat. No. 5,052,764 shows a catadioptric projection objective in which the edge of the concave mirror serves as system diaphragm for limiting the usable numerical aperture. The diaphragm diameter is defined by the mirror diameter.